1. Field of the Invention
This invention relates to bipolar transistors and more particularly to a lateral bipolar transistor useful alone or in combination with other transistors in integrated circuit form.
2. Description of the Prior Art
In the prior art lateral PNP bipolar transistors have been fabricated alone or with vertical NPN transistors. The lateral bipolar structure comprises two regions, emitter and collector, spaced apart of a first conductivity type with a third region, base, of a second conductivity type sandwiched therebetween. The separation between the first and second region determines the base width of the transistor. If the emitter current from the emitter region is injected along the total junction between the emitter and base, then the lateral injected current opposite the collector will participate in transistor action. The lateral emitter current injected away from the collector and the emitter current injected vertically transverse to the collector direction will most likely not participate in transistor action. The vertical emitter current is large due to the large horizontal junction area of the emitter region. A large portion of the injected emitter current is lost to recombination in the base region if not in the proximity of the collector. Consequently, the current gain or .alpha. of the present lateral transistor is low. The current gain .alpha. is determined by dividing the collector current I.sub.C by the emitter current I.sub.E.
In typical lateral PNP transistors, the component of current injected vertically from the emitter can be equal to or larger than the component injected laterally towards the collector because of the difference in horizontal to vertical junction area of the emitter. This is true even if the geometry permits a double sided emitter. In addition, if the emitter region is formed by diffusion into the base, the diffusion profile is sloped on the sides or edges which provides only a very small emitter junction area where the base width is narrowest. Transistor efficiency is further degraded by an emitter junction area with non-uniform base width. The frequency response of a lateral PNP transistor is determined by the diffusion constant D divided by the square of the diffusion length L. The non-uniform base width for a lateral PNP transistor provides longer diffusion lengths for the carriers which, in turn, reduces the frequency response f.sub.t.
One example of a lateral PNP transistor which may be formed with vertical NPN devices on the same integrated circuit chip is described in U.S. Pat. No. 3,197,710 which issued on July 27, 1965 to H. C. Lin entitled "Complementary Transistor Structure" which is assigned to the assignee herein. In U.S. Pat. No. 3,197,710 a complementary bipolar transistor structure is shown comprising a lateral PNP transistor with a vertically diffused NPN transistor such as shown in FIGS. 6 and 7. Both NPN and PNP transistors are made concurrently through a sequence of masking and diffusion steps. The PNP transistor structure shows an emitter as a dot diffused into an N region. The collector is formed by diffusing a ring or doughnut around the emitter having a constant space therebetween. The space between is the base of the PNP transistor and has constant base width in all directions radially as a function of depth from the emitter. The lateral PNP transistor shown in FIG. 6 or 7 is superior to a vertical PNP diffused transistor because it requires a large number of process steps, particularly diffusion operations which require a high degree of control. If any one of the several diffusion steps is performed improperly, an unsatisfactory vertical PNP bipolar transistor results. As stated in column 2 of U.S. Pat No. 3,197,710 the lateral PNP transistor "will not, in general, have sufficient gain to adequately perform as a transistor amplifier. Hence, in practically all applications where a complementary pair of transistor amplifiers is desired, the lateral transistor structure serves as a phase inverter and its output is amplified by the more" conventional vertical NPN transistor.
A semiconductor resistor is described in U.S. Pat. No. 4,314,269 by K. Fujiki which issued on Feb. 2, 1982 and filed prior to the invention herein. The resistor is formed by a conductivity layer on an internal side surface of a groove formed in semiconductor material of opposite conductivity type than the layer.
It is, therefore, desirable to provide a lateral bipolar transistor structure that has high current gain and good frequency response.
It is further desirable to provide a lateral bipolar transistor structure having a large component of emitter current injected laterally because of a large vertical junction area of the emitter compared to the current injected vertically from the horizontal junction area of the emitter.
It is further desirable to provide a lateral bipolar transistor structure having a uniform base width between the vertical junction areas of the emitter and collector which is easily and precisely controlled.
It is further desirable to provide a lateral PNP structure which can be incorporated with a conventional vertical NPN bipolar transistor process to form both PNP and NPN transistors concurrently in a semiconductor substrate.